thumb|A mesocyclone (at left) in the Central Zone of the city of [[Piracicaba, in southeastern Brazil, on January 28, 2025]]
thumb|Supercell diagram with the mesocyclone rotation in red
A mesocyclone is a meso-gamma mesoscale (or storm scale) region of rotation (vortex), typically around in diameter, most often noticed on radar within thunderstorms. In the Northern Hemisphere, it is usually located in the right rear flank (back edge with respect to direction of movement) of a supercell, or often on the eastern, or leading, flank of a high-precipitation variety of supercell. The area overlaid by a mesocyclone’s circulation may be several miles (km) wide, but substantially larger than any tornado that may develop within it, and it is within mesocyclones that intense tornadoes form.
Description
Mesocyclones are medium-scale vortices of rising and converging air that circulate around a vertical axis. They are most often associated with a local region of low-pressure. Their rotation is (usually) in the same direction as low pressure systems in a given hemisphere: counter-clockwise in the northern, and clockwise in the southern hemisphere, with the only occasional exceptions being the smallest-scale mesocyclones. Mesoanticyclones that rotate in an opposite direction may accompany mesocyclones within a supercell but these tend to be weaker and often more transient than mesocyclones, which can be sustained for tens of minutes or hours, and also cyclically form in succession within a supercell. Mesoanticyclones are relatively common with left-moving supercells that split from parent supercells in certain vertical wind shear regimes.
A mesocyclone is usually a phenomenon that is difficult to observe directly. Visual evidence of rotation – such as curved inflow bands – may suggest the presence of a mesocyclone, but the cylinder of circulating air is often too large to be recognized when viewed from the ground, or may not carry clouds distinct enough from the surrounding calmer air to make the circulating air flow obvious.
Mesocyclones are identified by Doppler weather radar observations as a rotation signature which meets specific criteria for magnitude, vertical depth, and duration. On U.S. NEXRAD radar displays, algorithmically identified mesocyclones, such as by the mesocyclone detection algorithm (MDA), are typically highlighted by a yellow solid circle on the Doppler velocity display; other weather services may have other conventions.
Within thunderstorms
They are of greatest concern when contained within severe thunderstorms, since mesocyclones often occur together with updrafts in supercells, within which tornadoes may form near the interchange with a downdraft.
Mesocyclones are localized, approximately to in diameter within strong thunderstorms. This local tendency for rotation, or twisting, is what the updraft reorients, rather than a literal tube or vortex of rotating air. When an updraft forms in this environment, ascending air parcels encounter faster sheared air across height, which is entrained and turbulently mixed at the edge of the updraft, exchanging horizontal momentum. The rising air at the edge of the updraft speeds up sideways faster than it is moving inward, forcing inner slower air to then also move faster horizontally. Air parcels then begin to curve as they move towards and overshoot the updraft's center of low pressure, following into a spiral as the process repeats. As the air parcels curve they also rotate about their axis due to the wind shear's twisting motion. This curving, spiraling or rotating motion of the wind can exist without the air necessarily spinning as a vortex.
As the low-level mesocyclone continues to ingest horizontal vorticity, vorticity maximums or vortex patches (areas of slight rotation or transient vortices) may form alongside the boundary where the updraft and its downdrafts – the cool and moist forward flank downdraft (FFD) and the, often, warmer and more buoyant rear flank downdraft (RFD) – meet due to the interactions between the warmer and cooler air masses. Surges in the RFD often coincide with the consolidation of these vortex patches, and may lead to tornadogenesis as a result. This is visually indicated by the formation of a wall cloud or other low cloud structures near the surface as the updraft strengthens from its interactions with the RFD.
The gallery below shows the three stages of development of a mesocyclone and a view of the storm relative motion on radar of a mesocyclone-producing tornado over Greensburg, Kansas on 4 May 2007. The storm was in the process of producing an EF5 tornado at the time of the image.
<gallery mode="packed">
File:Meso-1.svg|Wind shear (red) sets air spinning (green).
File:Meso-2.svg|The updraft (blue) 'tips' the spinning air upright.
File:Meso-3.svg|The updraft then starts rotating.
File:Greensburg3 small.gif|Radar view of a mesocyclone. Note that at the time of this image, an EF5 tornado was on the ground.
</gallery>
Identification
The most reliable way to detect a mesocyclone is by Doppler weather radar. Nearby high values of opposite sign within velocity data are how they are detected.
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